Synthesis of Spherical Core-Shell Ni(OH) 2 -Ni ½ Mn ½ (OH) 2 Particles via a Continuously Stirred Tank Reactor

Abstract

Core-shell particles of Ni(OH) 2 (core, 80 mol%) and Ni 1 / 2 Mn 1 / 2 (OH) 2 (shell, 20 mol%) were synthesized using a continuously stirred tank reactor (CSTR). This hydroxide core-shell material can be used as a precursor to LiNiO 2 -LiNi 1 / 2 Mn 1 / 2 O 2 core-shell materials, a possible positive electrode material for lithium ion batteries. The reaction was completed with different synthesis conditions to improve the in-situ coating of Ni(OH) 2 particles with a shell of Ni 1 / 2 Mn 1 / 2 (OH) 2. Spherical and dense particles of Ni(OH) 2 with a closed coating of Ni 1 / 2 Mn 1 / 2 (OH) 2 were synthesized. It was determined that the pH and stirring rate of the reaction must be altered when switching from the core to the shell to produce an optimum shell coating. Without changing reaction conditions some of the shell materials form new particles instead of coating existing particles. The shell and core compositions were verified with powder X-ray diffraction, scanning electron microscopy and ICP-OES elemental analysis. Lithium ion batteries for future electric vehicles will require an increase in energy density without sacrificing safety, lifetime or cost. One approach researchers are using to reach these goals is to de-velop hybrid, core-shell positive electrode materials that exploit the benefits of different materials. Often, the core of the particle is of a material with superior energy density, and the shell of the particle has excellent safety and stability to improve safety as well as calen-dar and cycle life. 1–4 A major cause of poor lifetime in lithium ion batteries is parasitic reactions between the electrode materials and the electrolyte 5 and the shell composition can be selected to minimize such reactions. Mixed transition metal positive electrode materials are commonly synthesized using a two-step process. 6 First a mixed transition metal hydroxide or carbonate precursor is synthesized via a co-precipitation reaction in a continuously stirred tank reactor (CSTR) or via other processes. The composition of the hydroxide or carbonate product depends on the composition of the aqueous metal cation solution pumped into the reactor. If one wishes to make a core-shell material the procedure is altered by changing the solution composition and/or flow rates during the reaction to coincide with the desired core and shell metallic composition and the desired ratio of core to shell. Similar to other lithium ion battery positive electrode materials the precursor is then ground with a desired amount of lithium carbonate or lithium hydroxide and then heated at high temperature yielding a lithium transition metal oxide positive electrode material. It is of importance to verify that the shell composition is forming a closed shell layer on the existing core particles, instead of form-ing new particles. Without a closed shell around the core the syn-ergy of the core-shell motif is not obtained. It is also important that the precipitate of the shell maintains the spherical and dense mor-phology of the core secondary particles since the lithium transition metal oxide cathode material will maintain most of the morphology of the precursor. Van Bommel et al. have previously developed meth-ods to synthesize spherical and dense Ni(OH) 2 and Ni 1 / 2 Mn 1 / 2 (OH) 2 and Ni 1 3